This document is obsolete.
This specification is not being actively maintained, and should not be used as a guide for implementations. It may be revived in the future, but for now should be considered obsolete. If you have questions or comments on this specification, please send an email to the CSS Working Group's mailing list at www-style@w3.org. (Before sending mail for the first time, you have to subscribe at http://lists.w3.org/Archives/Public/www-style/.)

Abstract

The intent of this document is to provide a set of detailed
algorithms which describe certain table layout behaviors that are common to most
major browsers today and propose additional standard behaviors in order to
achieve full browser interoperability in table layout. The Table layout
algorithm is one of the most used and most difficult to understand features in
HTML, as there are no common behavior descriptions in either HTML or CSS. The
goal of this document is to introduce a focused discussion around a unified
table behavior with the goal of getting agreement on a set of scenarios for
integration into the CSS 3 Tables specification.

This document is not complete but it tries to capture the
most common scenarios in which browsers have achieved interoperability. Our
hope is that this document is seen as a starting point to provide at least a
partial definition of table layout behavior, ten years after its introduction
into the market.

Captions in visual formatting model

Captions above or below a 'table' element are formatted very
much as if they were a block element before or after the table, except that (1)
they inherit inheritable properties from the table, (2) they are not considered
to be a block box for the purposes of any 'compact' or 'run-in' element that
may precede the table, and (3) they use the width of the anonymous box as input
for its available width calculations.

The width of the anonymous box is the border-edge width of
the table box inside it, as described by CSS 2.1 section 17.4. Percentages on
'width' and 'height' properties of the table are relative to the anonymous
box's containing block, not the anonymous box itself.

This algorithm follows the CSS 2.1 specification of the
caption-side property, where the only possible values are top, bottom and
inherit. CSS 3 will also add left and right. Thus in the algorithm
presented here, the caption width will not affect the table auto layout
algorithm in any way. However, the output of the table auto layout algorithm
may affect the sizing of table caption, since it determines the anonymous table
box size.

In
the following example, the 'caption-side' property places captions on top of tables.
The caption will be as wide as the anonymous box of the table, and caption text
will be left-justified.

caption { caption-side: top;

width: auto;

text-align: left }

Example of
the visual model:

Note: CSS3
might want to define a property that enables the developer to use the table's
parent width instead of the anonymous box width as an input in the caption
width algorithm.

Considerations for the caption width algorithm

Caption has either auto width
or a specified width unit value.

Padding, margin and border
should behave like a normal block box and are covered by the CSS Box Model
calculations.

Inputs

Table used width

Caption specified width (if
any)

Output

Caption's used width

Naming conventions

Name

Explanation

captionSpecifiedWidth

Input provided by the width property.

captionUsedWidth

Output that determines the width of the caption algorithm.

contentMinWidth

The minimal unit of breakable text (in most cases, the smallest
word in the caption).

tableUsedWidth

Input provided by table auto layout algorithm. This is the
width actually used by the table.

Algorithm

The
caption sizing algorithm can be summarized like this (ignoring possible constraints
set by the min-width and max-width properties)[1]:

If 'width'
is not 'auto' and 'border-left-width' + 'padding-left' + 'width' +
'padding-right' + 'border-right-width' + scrollbar width (if any) (plus any of
'margin-left' or 'margin-right' that are not 'auto') is larger than the width
of the tableUsedWidth, then any 'auto' values for 'margin-left' or
'margin-right' are, for the following rules, treated as zero.

If
all of the above have a computed value other than 'auto', the values are said
to be "over-constrained" and one of the used values will have to be
different from its computed value. If the 'direction' property of the containing block has the
value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so
as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.

If
there is exactly one value specified as 'auto', its used value follows from
the equality.

If 'width' is set to 'auto', all other 'auto' values
become '0' and 'width' follows from the resulting equality.

If both 'margin-left' and 'margin-right' are 'auto', their used values are equal.
This horizontally centers the element with respect to the edges of the
tableUsedWidth.

The
"scrollbar width" value is only relevant if the user agent uses a
scrollbar as its scrolling mechanism. See the definition of the 'overflow' property.

Automatic Table Layout Algorithm Overview

The design of the following algorithm is based upon the CSS
2.1 recommended algorithm (http://www.w3.org/TR/CSS21/tables.html#width-layout),
the HTML 4.1 recommended algorithm
(http://www.w3.org/TR/REC-html40/appendix/notes.html#h-B.5.2 ), and an
interoperability analysis of Internet Explorer 7, Firefox 3 Alpha, Opera 9,
and Safari 3 Beta. The goal of this design is to stick to the CSS 2.1 and
HTML 4.1 recommendations as close as possible while also accommodating the
common behaviors of the major browsers.

The algorithm proposed in this document uses several types of
inputs: the intrinsic characteristics of table cells (i.e., the minimum
and maximum widths of their content), the value of each cells column-spanning
(colspan) attribute (if any is set), and the specified width, minimum width
(min-width) and maximum width (max-width) of each table cell and the table
itself. The output of the algorithm is the final used width of the table and
each of its columns.

List of scenarios handled by this algorithm

This document is considering the following author inputs
into the algorithm:

Cell

Unit value

Percentage

Auto

min-width

N/A*

max-width

N/A*

Width

* Due to the current lack of implementations, percentage
values are not defined

Col/ColGroup

Unit value

Percentage

Auto

min-width

Cascaded **

Cascaded

Cascaded

max-width

Cascaded

Cascaded

Cascaded

Width

Cascaded

Cascaded

Cascaded

** A Cell value will overwrite Col value, which will overwrite
ColGroup as part of the cascade. This is an alternative proposal to the current
CSS2.1 definition

Table

Unit value

Percentage

Auto

min-width

max-width

Width

The output
will be:

The tables final used width

Each columns final used width

This will enable the following scenarios:

Table with the above specified inputs

Column-spanning scenarios, where the width on column spans can be expressed
in a unit value, auto or percentages

Simple nested table scenarios, as long as the intrinsic content width
can be determined

Still open issues:

Issue #1: Overlapping
spans (these work but do not produce an exact result)

Issue #2: Border
width(not taken into account, as it should be an easy addition)

Issue #3: Cell
padding (in both specified unit and percent values)

Issue #4: Cellspacing
/ border-spacing

Algorithm Summary

Table auto layout calculation is achieved with the following
steps.

1.Calculate each cells minimum width, preferred width and preferred
percent by examining the cells content and its width/min-with/max-width
properties.

2.Identify the minimum width, preferred width and preferred percent of
each column in the table by finding the largest minimum width, largest
preferred width and largest preferred percent among all cells in the column. In
this step, the algorithm first goes over all cells with colspan equal to one,
and then distributes the minimum width, preferred width and preferred percent
of each cell with colspan > 1 among the columns it spans over.

4.Determine the tables final used width by attempting to satisfy
each columns preferred width (or preferred percent if it has one) to the
greatest extent possible.

5.Decide how to distribute the tables final used width among all the
columns. During this process, columns with specified percentage width are
given first priority, followed by the columns with specified unit value width.
Columns with unspecified width (or width set to auto) has the lowest
priority.

6.Distribute any rounding errors introduced during distribution in
the previous step and adjust each columns width accordingly. (This step is
currently not part of this proposal.)

What follows is the proposed algorithm in finer detail.

We will color code
Terms that we will introduce for the first time.

1.Calculate each cells minimum and preferred width

This step calculates table width
limitation and preference at the cell level. These values are computed for each
cell based on its intrinsic minimum and preferred widths and its specified
width, min-width and max-width (if any).

Each cells minimum width sets the lower limit for that
cell. (A cell can never be narrower than its minimum width.)

This is a design decision drawn from a backwards
compatibility / interoperability analysis. It could be argued that display:
table-cell does have an overflow property and that it is thus not necessary to
always honor minimum width. The algorithm here, however, suggests ignoring
overflow property on table cell in auto layout table and respect minimum width
for best backward compatibility and readability. This of course, is open for
discussion.

The cells minimum width is calculated before the cells
preferred width, so that the minimum width can set a lower limit for the
preferred width.

Each cells preferred width is
the width the algorithm tries its best to satisfy. The preferred width is not
an upper limit of the cell because other cells in the same column and the
tables overall preferred width could cause the cell to be assigned a width
exceeding this value.

1.4Cell Preferred Percent (cellPreferredPercent)

cellPreferredPercent =
cellSpecifiedWidth in percent

. . .where cellPreferredPercent
is only defined when the cell has specified width in percent.

Issues

Issue #5: Specified percentage min-width and max-width are not
taken into consideration here. Their impact on the algorithm will be defined in
a future version of the document. Step 1.3 presents itself as a good place to
add in their impact

Issue #6: At the current stage, the algorithm hasnt considered any padding,
borders or border-spacing. These things will impact the calculation of
cellMinWidth / cellMaxWidth according to the CSS box model and can be added in
a future version of this document

2Identify
the minimum width, preferred width and preferredpercent of each
column

In this step, the algorithm collects
the width limitation and preference on a cell level and aggregates them up to
the column level. The goal of this step is to identify the minimum width,
preferred width and preferred percent of each column.

The algorithm first calculates
columnMinWidth, columnPreferredWidth, columnPreferredPercent based solely on
cells with colspan ==1. It then adjusts these three values with the minimum,
preferred width and preferred percent of colspan > 1 cells.

columnMinWidth = max (cellMinWidth of all
colspan == 1 cells in the column)

The columns minimum width sets the lower limit on columns
used width. The column can never be smaller than its minimum width, in order to
prevent overflow of the text into neighboring cells. This is a design decision
to enable better readability.

2.1.2Column Preferred Width (columnPreferredWidth)

columnPreferredWidth = max
(cellPreferredWidth of all colspan ==1 cells in the column)

The columns preferred width defines the
width which the algorithm tries to honor.

2.1.3Column Preferred Percent (columnPreferredPercent)

Only columns with at least one cell with specified
width in percent will have a columnPreferredPercent value It is found by taking
the largest specified percent cell width:

columnPreferredPercent = max (cellPreferredPercent of
all colspan == 1cells in the column)

To better understand the behavior, the algorithm defines
three types of columns:

a.auto column: all cells in the column have either width: auto
or no specified width

b.specified unit value column: at least one cell in the column has
a specified width in unit value and no cell has specified width in percent.
(Specified width can be cascaded or inline.)

c.specified percent column: at least one cell in the column has a
specified width in percent

A specified percentage cell has a higher priority than
a specified unit value cell. For instance, one specified percentage width cell
will make the column a specified percent column even if all the other cells
have specified unit value width.

Only non-negative percentage values are taken into
consideration. It may happen that the percentage value exceeds 100%, or the sum
of all columns specified percentage value exceeds 100%. Cases like these will
be handled in step 5, when columns are assigned their final used widths.

The proposed algorithm handles cell with colspan >
1 one by one without any optimization for overlapping colspan scenarios. An
optimized algorithm will be proposed in a future version of this document.

The algorithm first checks to see if any cells spanning
multiple columns will affect the minimum width, preferred width, or preferred
percent widths of the columns they span. If such is the case, theses widths of
the multi-column cell are distributed among the corresponding width values of
each of the columns it spans. There are several ways how this can be done, but
the method outlined in this spec was chosen based on browser interoperability.

In general, the algorithm will distribute the impact
evenly or proportionally with either (1) each columns preferred width, (2) the
difference between each columns preferred width and minimum width (i.e.,
preferred width min width), or (3) each columns minimum width,
depending on the case. When the algorithm distributes the impact of a
multi-column cell, the columns minimum and preferred widths are the sole
contributors to the distributionits specified width is not directly figured
in. This is because the impact of a multi-column cells specified unit
value width has already been calculated into its columns preferred width. The
impact of a multi-column cells specified percentage width will be distributed
proportionally to each non-percentage column that is spanned by the
multi-column cell.

When calculating the impact of the multi-column cell on the
columns, the algorithm adjusts the columns minimum width first, preferred
width second and preferred percent last.This follows the guidelines for
min/max width calculations of CSS2.1 (section 10.4). This enables the preferred
width to be restricted by the minimum width. The distribution of a multi-column
cells preferred percentage width will now depend on the newly adjusted minimum
and preferred widths of its columns.

Issues

Issue #7: As mentioned above, the algorithm hasnt considered any
padding, borders or border-spacing. These inputs will impact the calculation of
columnMinWidth, columnPreferredWidth and columnSpecifiedWidth according to CSS
box model, and can be added in future version of this document

3.Calculate
tableMinWidth/tablePreferredWidth

This step calculates the tables
minimum and preferred widths.

3.1Table Minimum Width (tableMinWidth)

tableMinWidth =
sum(columnMinWidth)

3.2Table Preferred Width (tablePreferredWidth)

As the following steps demonstrate,
tablePreferredWidth is only used in the absence of a table specified width,
when the table is sized according to the shrink-to-fit equation. The
calculation of tablePreferredWidth could therefore be ignored in some cases to
optimize performance.

There are three cases to address
when calculating tablePreferredWidth:

a.When there is no specified percent columns,

The preferred table width is simply
the sum of columnPreferredWidths.

tablePreferredWidth =
sum(columnPreferredWidth)

b.When there is at least one specified percent column,

When there are specified percent
columns, the algorithm uses the largest column
columnPreferredWidth-to-columnPreferredPercent ratio to determine the table
width preferred by the percent columns:

Next the algorithm pretends that
the non-percent columns (i.e., the auto and specified unit value columns) were
just one big percent column with a preferred percentage equal to (100% -
percentColumnTotalSpecifiedPercent), and uses that value to find the ratio of
columnPreferredWidth-to-columnPreferredPercent, which represents the table
preferred width of the non-percent columns:

When the sum
of preferred percentages exceeds 100%, the algorithm recommends that the
browser ignore all specified percentages coming after the sum reaches 100% and
regard them as having 0% specified percent width (though this doesnt
necessarily prevent them from being rendered).

When there
are only specified percent columns and the sum of the percent is smaller than
100%, step 5 will provide a solution to distribute extra table used width.

c.When percent columns are demanding the full table width (100%) and there
are also non-percent columns

Because the algorithm does not allow
the sum of specified percentages to be larger than 100%, a special case might
arise where the percent columns are demanding the full table width (100%) but
there are also non-percent columns which require at least their minimumwidth. In this case, the tables preferred
width could never be satisfied. The algorithm treats tables preferred width as
infinitely large

tablePreferredWidth = INFINITE

Issue:

Issue #8: Again
in this step, the CSS padding, margin, border and border-spacing
properties and HTML cellpadding and cellspacing attributes will all have an
impact on the calculations, and need to be detailed in a future version of this
document

4.Determine
the tables final used width (tableUsedWidth)

This step must differentiate two
cases: (1) when the table has a specified width, and (2) when the table is
auto-sized.

4.1When
the table has a specified width

In this case, the tables specified
width is taken as its used width, unless that width is smaller than the tables
minimum width, in which case its minimum width becomes the tables final used
width. If the table has a specified width in percentage, the algorithm converts
this to a unit value based on the tables available width.

. . . where tableAvailableWidth
is the width of the tables containing block.

4.3Adjust
tableUsedWidth against specified min-width/max-width

This
algorithm strictly follows the min-width/max-width rule defined in the CSS 2.1
spec (section 10.4) to adjust the tables used width by its specified
min-width and max-width as needed. When these values are in percentages,
the algorithm converts them to a unit value based on the tables available
width.

4.3.1Check against specified max-width

If tableUsedWidth is larger than the
specified max-width, use max-width as the specified width and recalculate
tableUsedWidth with formula in 4.1.

4.3.2Check against specified min-width

If tableUsedWidth is smaller than the
specified min-width, use min-width as the specified width and recalculate
tableUsedWidth with formula in 4.1.

Issue

Issue #9: Border,
margin and padding will have an impact on this step (as is specified in the CSS
box model, and this needs to be accounted for in a future version of this
document

5.Distribute
the tables final used width among all of the columns

Ideally, each column should get
its preferred width. However, tableUsedWidth may not be big enough or small
enough for the satisfactory result. We follow three rules here when determining
column used width (columnUsedWidth)

Rule No.1 When assigning
preferred widths, specified percent columns have a higher priority than
specified unit value columns, which have a higher priority than auto columns.

Rule No.2- Columns in the
same category follow the same distribution method. For example, they all get
columnMinWidth or they all get columnPreferredWidth, etc.

There is one exception to this
rule. When applying columnPreferredPercent on a percent column, the computed
unit value may be smaller than columnMinWidth. In this case, the column will be
assigned its columnMinWidth though the percent column group as a whole is still
regarded as being assigned the columnPreferredWidth.

Rule No.3 The sum of
columnUsedWidth should be equal to tableUsedWidth

A value between columnMinWidth and
columnPreferredWidth / columnPreferredPercent

d.columnPreferredWidth + delta or columnPreferredPercent + delta

A value larger than
columnPreferredWidth / columnPreferredPercent

Based upon rule No.1, the
algorithm allows for the following combinations:

specified percent column

specified unit value column

auto column

1

columnMinWidth

columnMinWidth

columnMinWidth

2

columnMinWidth+delta

columnMinWidth

columnMinWidth

3

columnPreferredPercent

columnMinWidth

columnMinWidth

4

columnPreferredPercent

columnMinWidth+delta

columnMinWidth

5

columnPreferredPercent

columnPreferredWidth

columnMinWidth

6

columnPreferredPercent

columnPreferredWidth

columnMinWidth+delta

7

columnPreferredPercent

columnPreferredWidth

columnPerferredWidth

8

columnPreferredPercent

columnPreferredWidth

columnPreferredWidth + delta

9

columnPreferredPercent

columnPreferredWidth + delta

DOES NOT EXIST

10

columnPreferredPercent+delta

DOES NOT EXIST

DOES NOT EXIST

Detailed explanations of these
distribution combinations are provided at the end of this section. In the absence
of any of the three column types, the algorithm behaves as if the absent
columns preferred and minimum widths are zero.

Assigment of values

In order to fulfill Rule No.3, so
that the sum of the column used widths completely fill the used width of the
table, we apply the following assignments:

a.Assign combination 1 to each column

If sum(columnUsedWidth) is
equal to tableUsedWidth, use combination 1.

Otherwise
continue

b.Assign combination 3 to each column

If sum(columnUsedWidth) is
equal to tableUsedWidth, use combination 3.

Otherwise, if the sum is
smaller, use combination 2.

Otherwise,
continue.

c.Assign combination 5 to each column

If sum(columnUsedWidth) is
equal to tableUsedWidth, use combination 5.

Otherwise, if the sum is
smaller, use combination 4.

Otherwise,
continue.

d.assign combination 7 to each column.

If sum(columnUsedWidth) is
equal to tableUsedWidth, use combination 7.

Otherwise, if the sum is
smaller, use combination 6.

Otherwise,
continue.

e.If there is any auto column, use combination 8.

otherwise, if there is any
specified unit value column, use combination 9.

otherwise, if there is any
specified percent column, use combination 10.

Please note that this decision
method requires several passes of column width assignments, and UAs are free to
adopt a different algorithm requiring fewer steps.

What follows are more detailed
explanations of each distribution combination.

Combination 1

All columns get their
columnMinWidth.

specified percent column

specified unit value column

auto column

1

columnMinWidth

columnMinWidth

columnMinWidth

Combination 2

Specified unit value column and auto
column get their columnMinWidth. The rest of tableUsedWidth should be
distributed to specified percent column in proportion to their
columnPreferredPercent.

specified percent column

specified unit value column

auto column

2

columnMinWidth+delta

columnMinWidth

columnMinWidth

Combination 3

Specified unit value column and
auto column get their columnMinWidth. Specified percent columns get their
columnPreferredPercent.

specified percent column

specified unit value column

auto column

3

columnPreferredPercent

columnMinWidth

columnMinWidth

Combination 4

Specified percent columns get
their columnPreferredPercent and auto columns get their columnMinWidth.

Specified Unit Value columns are
first assigned their columnMinWidth. The remaining of tableUsedWidth is
distributed to specified unit value columns in proportion to the difference
between columnPreferredWidth and columnMinWidth (columnPreferredWidth - columnMinWidth).

specified percent column

specified unit value column

auto column

4

columnPreferredPercent

columnMinWidth+delta

columnMinWidth

For instance, suppose there are two
specified unit value columns and the remaining tableUsedWidth is 40px. Here is
how we determine each columns used width.

column 1

column 2

columnMinWidth

5px

70px

columnPreferredWidth

15px

100px

columnPreferredWidth - columnMinWidth

10px

30px

delta

10px (= 40px * 1/4)

30px (= 40px * 3/4)

columnUsedWidth

15px (=5px + 10px)

100px (= 70px + 30px)

Combination 5

Specified percent columns get their
columnPreferredPercent. Specified unit value columns get their
columnPreferredWidth and auto columns get columnMinWidth.

specified percent column

specified unit value column

auto column

5

columnPreferredPercent

columnPreferredWidth

columnMinWidth

Combination 6

Specified percent columns get
their columnPreferredPercent and Specified unit value columns get their
columnPreferredWidth

Auto columns are first assigned
their columnMinWidth. The remaining of tableUsedWidth is distributed to auto
columns in proportion to the difference between columnPreferredWidth and
columnMinWidth (columnPreferredWidth - columnMinWidth). (The distribution is
very similar to combination 4).

specified percent column

specified unit value column

auto column

6

columnPreferredPercent

columnPreferredWidth

columnMinWidth+delta

Combination 7

Specified percent columns get
their columnPreferredPercent. Specified unit value columns and auto columns all
get their columnPreferredWidth.

specified percent column

specified unit value column

auto column

7

columnPreferredPercent

columnPreferredWidth

columnPerferredWidth

Combination 8

Specified percent columns get their
column PreferredPercent and Specified unit value columns get their
columnPreferredWidth.

The remaining tableUsedWidth is
distributed to auto columns in proportion to their columnPreferredWidth.

specified percent column

specified unit value column

auto column

8

columnPreferredPercent

columnPreferredWidth

columnPreferredWidth + delta

For instance, suppose there are
two auto columns and the remaining tableUsedWidth is 160px. Here is how we determine
each columns used width.

column 1

column 2

columnPreferredWidth

30px

90px

columUsedWidth

40px = 160 *

120px = 160 *3/4

Combination 9

This is used when there is no auto
columns and the sum of columnPreferredPercent converted to unit value plus
columnPreferredWidth is smaller than tableUsedWidth. In this case, each
specified percent columns are assigned their columnPreferredPercent. The
remaining of tableUsedWidth is assigned to specified unit value columns in
proportion to their columnPreferredWidth.

specified percent column

specified unit value column

auto column

9

columnPreferredPercent

columnPreferredWidth + delta

DOES NOT EXIST

Combination 10

This is used when there is only
specified percent column and the sum of columnPreferredPercent converted to
unit value are smaller than tableUsedWidth. (It could only happen when the sum
of columnPreferredPercent is smaller than 100%). In this case, each specified
percent columns are assigned a width in proportion with their
columnPreferredPercent.

specified percent column

specified unit value column

auto column

10

columnPreferredPercent+delta

DOES NOT EXIST

DOES NOT EXIST

6.Distribute
any rounding errors

At this stage the UA only has to deal with rounding errors
that have arisen as part of multiplications/ divisions and other adjustments
during the calculations. These kinds of errors occur when the sum of the all
columns used widths is not equal to the tables used width. This document does
not propose guidelines on how to implement rounding errors. The Working Group
might decide to give a preferred treatment of rounding errors as part of this
document.

The idea is, for a given table width, to either assign one of
the following sets of column widths or a weighted average of two adjacent sets
of widths. It is not possible to assign values smaller than the smallest
set of widths. However, see below for handling the case of assigning
values larger than the largest set of widths. From smallest to largest,
these are:

1.Assign all columns their min width.

equivalent
to Combination 1

the
weighted average of 1 and 2 is equivalent to Combination 2

2.Assign all columns with percentage widths their percentage width, and
all other columns their min width.

equivalent to Combination 3

the weighted average of 2
and 3 is equivalent to Combination 4

3. Assign all columns with percentage widths their percentage width,
all columns with specified coordinate widths their pref width (since it doesn't
matter whether it's the largest contributor to the pref width that was the
specified contributor), and all other columns their min width.

equivalent to Combination 5

the weighted average of 3
and 4 is equivalent to Combination 6

4.Assign all columns with percentage widths their specified width, and all
other columns their pref width.

equivalent to Combination 7

If the table's width is *larger*
than what we would assign in (4), then we expand the columns:

a)if any columns without a specified coordinate width or percent width
have nonzero pref width, in proportion to pref width

equivalent
to Combination 8

b)otherwise, if any columns without percent width have nonzero pref width,
in proportion to pref width

equivalent
to Combination 9

c)otherwise, if any columns have nonzero percentage widths, in proportion
to the percentage widths

equivalent
to Combination 10

d)otherwise, equally.

It
is only possible when table is all empty. In that case, it doesnt make sense
to calculate columnUsedWidth, which should all be 0.